1. Field of the Invention
The invention relates to a position detection system for a body which changes position, comprising a series of markings made at a mutual distance from one another and a scanner unit, which is movable with respect to said series of markings, for scanning said markings, the series of markings or the scanner unit being connected to the body.
2. Description of the Related Art
An installation of this type is disclosed in German patent application 3,116,333. Said application describes a position detection system for determining the position of the piston rod of a piston-cylinder assembly. To this end a series of uniform markings located at equal distances from one another is made on the piston rod, which markings are read off by an optical sensor. With the aid of the optical sensor, the successive passage of the markings present on the piston rod, which is a measure of the distance over which the piston rod has moved, is recorded, so that the position of, for example, the end of the piston rod can be deduced therefrom. This is therefore a so-called incremental system, which has disadvantages.
Firstly, because of the incremental functioning, uninterrupted accurate monitoring of the movements of the piston rod during its change in position is particularly important. Malfunctions which occur during said change in position lead directly to an inaccurate determination of position. Such malfunctions can be, for example, the "missing" of the passage of one or more markings by the sensor, or a malfunction in, for example, the memory of the counting system which is coupled to the sensor and is used to keep count of how many markings have passed by the detector from the start of the series. Secondly, the permissible speed of movement of the piston rod is determined by the detection speed of the sensor used. For accurate positioning, this permissible speed is relatively low. Thirdly, the accuracy of the positioning is to a large extent determined by the dimensions of the markings and their mutual spacing. The finer the markings and the smaller their mutual spacing, the greater is the theoretical accuracy. However, in the case of very fine markings stringent demands are placed on the sensor and on the technology for applying the markings. Furthermore, the sensitivity to malfunctioning increases, for example as a result of soil depositing on the markings. For example, the determination of position to an accuracy of 3 .mu.m necessitates bars 2 .mu.m wide as markings, which dimensions are significantly smaller than those of the usual dust particles floating freely in the air. It is therefore possible to achieve good reliability in the case of accurate positioning with the aid of the known positioning installation only at relatively high cost.